EEL 5718 Computer Communications Chapter 4 Multiplexing and

EEL 5718 Computer Communications Chapter 4: Multiplexing and Switching

Outline • Multiplexing – – TDM FDM WDM CDM • Switching – – Circuit switching Message switching Packet switching Virtual circuit switching

Multiplexing • Multiplexing: sharing method of expensive network resources by multiple connections or information flows (to share the same link) • Resources: frequency (wavelength), time, code (a) (b) A A A B B B C C C A Trunk group MUX B C

Frequency Division Multiplexing (FDM) • Multiple flows or connections share the link over different carriers A – Used in telephony – Most commonly used f W 0 B 0 f W C 0 f W A B C f

Time Division Multiplexing (TDM) • Time sharing: (a) Each signal transmits 1 unit every 3 T seconds – round-robin – T 1 line: 24 voices A 1 A 2 0 T t 6 T 3 T B 1 B 2 t 6 T 3 T 0 T C 1 0 T C 2 t 6 T 3 T (b) Combined signal transmits 1 unit every T seconds. A 1 B 1 C 1 A 2 B 2 C 2 t 0 T 1 T 2 T 3 T 4 T 5 T 6 T

Wavelength Division Multiplexing (WDM) • Similar to FDM (one-to-one correspondence to frequency), commonly used in optical networks • One fiber line transmits multiple colors 1 2 m Optical MUX Optical de. MUX 1 2. m 1 2 Optical fiber m

Code Division Multiplexing (CDM) • Different connections or flows use different codes: orthogonal codes are used • Not exactly share the same link, commonly used in wireless systems: multiple users share the same channel

Switching • Long distance transmission is typically done over a network of switched nodes • Intermediate switching nodes not concerned with content of data • End devices are stations – Computer, terminal, phone, etc. • Data routed by being switched from node to node (a router is a more complicated switch)

Simple Switched Network

Switching Technologies • Circuit switching • Packet switching (including Message switching) • Virtual circuit switching

Circuit Switching • Dedicated communication path between two stations • Three phases – Setup (signaling exchange) – Transfer (communications service exchange) – Disconnect • Must have switching capacity and channel capacity to establish connection

Circuit Switching (cont) • Inefficient – Channel capacity dedicated for duration of connection – If no data, capacity will be wasted • • Set up (connection) takes time Once connected, transfer is transparent Developed for voice traffic (phone) Will be efficient if plenty of data are available!

Telephony: an example

Telecommunications Components • Subscriber – Devices attached to network • Local Loop – Subscriber loop – Connection to network • Exchange – Switching centers – End office - supports subscribers • Trunks – Branches between exchanges – Multiplexed

Pedestal local telephone office (LEC: Local Exchange Center) Distribution Frame Serving Area I/f distribution cable Serving Area I/f Switch feeder cable Figure 4. 33

Switches • Blocking – Only finite paths in networks – A network is unable to connect stations because all paths are in use – Blocking is possible: “all circuits are busy” – Used on voice systems è Short duration calls • Non-blocking – Permits all stations to connect (in pairs) at once, no blocking inside a switch – Used for some data connections (high speed)

Space Division Switching • Developed for analog environment • Separate physical paths • Crossbar switch – Number of cross-points grows as square of number of stations – Loss of cross-point prevents connection – Inefficient use of cross-points è All stations connected, only a few cross-points in use – Non-blocking

Crossbar Matrix

Multistage Switch • Reduce the number of cross-points • Increase the scalability • More than one path through network – Increase reliability • More complex control • May be blocking

Three Stage Switch

Time Division Switching • Partition low speed bit stream into pieces that share higher speed stream • e. g. TDM bus switching – based on synchronous time division multiplexing – Each station connects through controlled gates to high speed bus – Time slot allows small amount of data onto bus – Another line’s gate is enabled for output at the same time

Time-slot Interexchange (TSI) 1 From • TDM 2 De. MUX 24 23 2 2 1 24 1 Read slots in permuted order 23 1 2 24 To TDM MUX Figure 4. 25

Routing • A network can be regarded as a grand switch • Many connections will need paths through more than one switch • Need to find a route: efficiency and resilience • Public telephone switches are a tree structure – Static routing uses the same approach all the time • Dynamic routing allows for changes in routing depending on traffic – Uses a peer structure for nodes

Control Signaling • • • Signaling for connection establishment Signaling for connection tearing down Signaling for communication transfer Signaling for Qo. S CCS (Common Channel Signaling), SSN 7 Common practice: control signaling network overlays the data network (cellular networks)

Packet Switching • Circuit switching designed for voice – Resources dedicated to a particular call – Much of the time a data connection is idle – Data rate is fixed • Data streams are bursty, too much idle periods if circuit switching is used • Rate for data transmissions may be much higher than voice transmissions

Packet Switching • Similar to mail system • Data transmitted in small packets – Longer messages split into series of packets – Each packet contains a portion of user data plus some control info • Control info – Routing (addressing) info • Packets are received, stored briefly (buffered) and past on to the next node – Store and forward

Use of Packets

Switching Technique • Message switching: a special type of packet switching, basic PDU is message-based, not efficient when the message is long • Packet switching: station breaks long message into packets • Packets sent one at a time to the network • Packets handled in two ways – Datagram – Virtual circuit

Datagram • • • Each packet treated independently Packets can take any practical route Packets may arrive out of order Packets may be missing Up to the receiver to re-order packets and recover from missing packets

Virtual Circuit • Preplanned route established before any packets sent • Call request and call accept packets establish connection (handshake) • Each packet contains a virtual circuit identifier instead of destination address • No routing decisions required for each packet • Clear request to drop circuit • Not a dedicated path, rather a shared path

Virtual Circuit Switching vs Circuit Switching • Circuit switching setups a DEDICATED path • VC switching setups a path, however, a shared path, links along the path may be shared by other circuits • Remark: In VC, we do not need the overhead as in packet switching, however, VC is not reliable, i. e. , “packets” flowing on VC may be dropped! The control info are used for error recovery. • ATM is a special type of packet switching using virtual switching modes

Circuit v Packet Switching • Performance – Propagation delay – Transmission time – Node delay (queueing delay)

Switching operations

Suggested Reading • Textbook Chapter 4 • Stallings Chapter 8 and 10 • Tanenbaum Chapter 2 (2. 4, 2. 6)